Stackable storage unit



FIG. 1 is a bottom perspective view of stackable storage unit, showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines in the drawings illustrate the portions of the stackable storage unit, which form no part of the claimed design. 

CLAIM The ornamental design for stackable storage unit, as shown and described. 